Under Windows, use an Interlocked function for Add that is not only  available on Itanium.
Under Windows, use an Interlocked function for Add that is not only 
available on Itanium.

* subversion/libsvn_subr/svn_named_atomic.c
  (synched_add): use InterlockedExchangeAdd64 instead of InterlockedAdd64

Found by: rhuijben

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion